Port to event controllers

As preparation for porting to GTK4, we can adopt event controllers to stop using the GtkWidget::event signal.

This Merge Request ports the whole codebase with 2 notable exceptions:

  1. The scroll events in NautilusFilesView, for zooming, because GtkEventControllerScroll doesn't return a boolean in GTK3. I have a patch for this but it's GTK4-only.
  2. The canvas view, which is obsolete and not expected to be ported to GTK4 (see #319 (closed))

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